Summary of work: Nurrl is a transcription factor that structurally resembles members of the superfamily of nuclear hormone receptors. DNA binding domains of these receptors contain two zinc fingers and usually a ligand and dimerization domain confined to the COOH-terminus. Since a ligand for Nurrl has not been identified, it is also considered an orphan receptor. Nurrl expression appears to be predominantly brain specific detected in embryonic stages with the peak expression at birth or soon thereafter. Most striking is its expression in the substantia nigra (SN) and ventral tegmental area (VTA) of the midbrain, which contains most of the brain dopamine (DA). To ascertain the function of the orphan receptor, we generated Nurrl-null mice by homologous recombination. In Nurrl-null mice the concentration of DA was decreased by 90% when compared with the wild type. The results pointed to DA deficiency from a failure to develop the dopaminergic phenotype. We have investigated further the identity, cell fate and the presence of neuronal projections of those neurons identified by neuron-nuclear specific (NeuN) immunereactivity in the ventral midbrain of the Nurrl deficient neonates. In situ hybridization using untranslated exon 1 and 2 of Nurrl as a probe, specifically labeled the SN and VTA in Nurrl mutant neonates in contrast to the probe derived from exon 3 which was replaced in the mutant by neomycin gene. The message for the cholecystokinin (CCK) like peptide previously shown to be coexpressed with tyrosine hydroxylase, the rate limiting enzyme in DA biosynthesis, in a population of mesencephalic neurons was predominately detected in the VTA. Using the TUNEL reaction we have not detected any difference in the number of apopotic cells in the SN and VTA between Nurrl deficient and wild type neonates. Since the survival of neurons during development depends on interaction with their targets, we investigated whether NeuN, CCK and Nurrl exon 1 and 2 positive neurons in mutant neonates would project normally to the striatum. Anterograde and retrograde tracing with a fluorescent tracer revealed that nigrostriatal and striatonigral neuronal projections were well established. Taken together, our results imply that in the SN and VTA the transcription factor Nurrl controls dopaminergic neuron phenotype rather than cell fate or survival of these neurons. Clinically, the finding implies that the activation of Nurrl may be therapeutically useful for Parkinson's patients. Presently, it is not known what factors are responsible for Nurrl selective expression. To understand the mechanism controlling Nurrl expression, we have created transgenic mice containing about 5, 3, and 1 kilobases of the Nurrl gene 5' flanking region driving the expression of the reporter gene LacZ. These constructs have been established in the germ line. The progeny of these mice will be analyzed by staining brain sections for LacZ expression, to determine which Nurrl genomic fragment is sufficient for tissue and cell-type specific expression.